SOLVING EQUATIONS WITH ABSOLUTE VALUES ON BOTH SIDES

The picture shown below explains how to solve the equations in which we have absolute value sign on both sides.

Example 1 :

Solve for x :

|x - 3| = |3x + 2|

Solution :

Based on the idea given above, we have

x - 3 = (3x + 2) 

x - 3 = 3x + 2

- 5 = 2x

- 5/2 = x


(or)


x - 3 = -(3x + 2) 

x - 3 = - 3x - 2

- 1 = - 4x

1/4 = x

Justify and evaluation :

Substitute x = -5/2 and x = 1/4 in the given absolute value equation. 

|-5/2 - 3| = |3(-5/2) + 2|

|-11/2| = |-15/2 + 2|

|-11/2| = |-11/2|

11/2 = 11/2

|1/4 - 3| = |3(1/4) + 2|

|-11/4| = |3/4 + 2|

|11/4| = |11/4|

11/4 = 11/4

Substituting x = -5/2 and x = 1/4 into the original equation results in true statements.

Both the answers x = -5/2 and x = 1/4 are correct and acceptable.

Problem 2 :

Solve for x :

|x - 7| = |2x - 2|

Solution :

Based on the idea given above, we have

x - 7 = 2x - 2

x - 7 = 2x - 2

- 5 = x


(or)


x - 7 = - (2x - 2) 

x - 7 = -2x + 2

3x = 9

x = 3

Justify and evaluation :

Substitute x = -5 and x = 3 in the given absolute value equation.

x = -5

|-5 - 7| = |2(-5) - 2|

|-12| = |-10 - 2|

|-12| = |-12|

12 = 12

x = 3

|3 - 7| = |2(3) - 2|

|-4| = |6 - 2|

|-4| = |4|

4 = 4

Substituting x = -5 and x = 3 into the original equation results in true statements.

Both the answers x = -5 and x = 3 are correct and acceptable.

Problem 3 :

Solve for z :

|2z + 5| = |2z - 1|

Solution :

Based on the idea given above, we have

2z + 5 = 2z - 1

5 = -1

5 = -1

The above statement is false. 

No solution here.


(or)


2z + 5 = - (2z - 1) 

2z + 5 = -2z + 1

4 = -4z

-1 = z

Justify and evaluation :

Substitute z = -1 in the given absolute value equation.

|2(-1) + 5| = |2(-1) - 1|

|-2 + 5| = |-2 - 1|

|3| = |-3|

3 = 3

Substituting z = -1 into the original equation results in true statement. 

So, the answer z  = -1 is correct and acceptable.

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